1. Field of the Invention
The present invention concerns an electrochemical cell in which the negative active material is based on an alkali or alkaline earth metal and the positive material is a liquid oxyhalide such as thionyl chloride and also constitutes the electrolyte solvent, the electrolyte possibly comprising other cosolvents.
2. Description of the Prior Art
When a cell of this kind is not being discharged, the liquid substance serving as the positive material and electrolyte solvent reacts with the metal of the negative electrode as a result of which a protective surface film is formed on this electrode. The presence of a film of this kind constitutes a disadvantage since it may cause a "voltage rise delay" at the start of discharge; in other words, the normal operating voltage is not obtained until the end of a certain time delay. This phemonenon is more accentuated with longer storage times and higher storage temperatures.
Various solutions have already been put forward to minimize this "voltage rise delay".
For example, U.S. Pat. No. 4,309,940 published Jan. 5, 1982 describes a lithium-thionyl chloride cell of which the electrolyte solute is a complex salt resulting from the action of lithium chloride LiCl on aluminum chloride AlCl.sub.3, the latter being totally neutralized by LiCl to form LiClCl.sub.4. The patent provides for the addition to the electrolyte of a certain quantity of sulfur dioxide SO.sub.2, the molar ratio of AlCl.sub.3 :SO.sub.2 being between 0.9 and 1.5:1. Thus this method entails the addition of significant concentrations of sulfur dioxide, which may result in cell pressurization phenomena and practical difficulties of implementation.
There are proposed in U.S. Pat. No. 4,228,229 published Oct. 14, 1980 and French Pat. No. 2 485 271 published Dec. 24, 1981 other techniques employing partial or total neutralization of solutions of aluminum chloride by Lewis bases other than LiCl, Li.sub.2 O or Li.sub.2 CO.sub.3, for example. These techniques have made it possible to achieve a significant improvement as compared with the initial electrolyte prepared by neutralization of a solution of AlCl.sub.3 with LiCl. They produce solutions also containing sulfur dioxide SO.sub.2. In this case, it is generated "in situ" during the neutralization of AlCl.sub.3 by the corresponding Lewis bases, but in proportions such that the molar ratio of AlCl.sub.3 :SO.sub.2 is greater than or equal to 2:1, acording to the degree of neutralization selected.
To give an example, and for reasons associated with cell self-discharge, use has been made of totally neutralized electrolyte (molar ratio of AlCl.sub.3 :SO.sub.2 =2:1) with a concentration of LiAlCl.sub.4 corresponding to 1.35 moles/l. The reaction used is, for example, as follows: EQU 2AlCl.sub.3 +Li.sub.2 O+SOCl.sub.2 .fwdarw.2LiAlCl.sub.4 +SO.sub.2
Although the solution explained hereinabove has resulted in significant improvements in terms of reduced voltage rise delays, it has nevertheless proven insufficient for a certain number of applications. An object of the present invention is to further reduce the voltage rise delay.